PRE-EMPTIVE RISK ASSESSMENT SHOULD BSE IN SMALL RUMINANTS BE FOUND UNDER DOMESTIC CONDITIONS.

Transcription

1 EUROPEAN COMMISSION HEALTH & CONSUMER PROTECTION DIRECTORATE-GENERAL Directorate B - Scientific Health Opinions Unit B1 - Monitoring and dissemination of scientific opinions Scientific Steering Committee OPINION PRE-EMPTIVE RISK ASSESSMENT SHOULD BSE IN SMALL RUMINANTS BE FOUND UNDER DOMESTIC CONDITIONS. ADOPTED BY THE SCIENTIFIC STEERING COMMITTEE AT ITS MEETING OF 8-9 FEBRUARY 2001

2 1 EXECUTIVE SUMMARY The European Commission requested the Scientific Steering Committee (SSC) to (1) pre-emptively carry out risk assessments corresponding to a range of circumstances (scenarios) should BSE be confirmed to occur in the sheep population under domestic conditions. (2) present an updated state of affairs regarding tissue infectivity distribution on the basis of the most recent results of the ongoing experiments on BSE in small ruminants, (3) identify and list the criteria to be fulfilled to conclude that the BSE/vCJD strain has been isolated from sheep 1. The SSC's opinion on the situation regarding BSE in small ruminants remains unchanged from the position adopted by the SSC on September 1998, namely that one "has to assume that BSE could have been introduced into the sheep and goat population because it has clearly been demonstrated [by experiments] that BSE can be orally transmitted to certain genotypes of small ruminants and because it is likely that BSE-contaminated MBM has been fed to some sheep and goats". There is no evidence to confirm that BSE is present in small ruminants. However, the extent of observations to detect its presence is very limited and we do not at present have adequate means (by monitoring, testing, etc.) to confirm beyond reasonable doubt a diagnosis of BSE in sheep directly or indirectly exposed to the BSE agent. The SSC considers that it cannot at this stage assess the risk to the human population from the possibility that BSE exists in small ruminants under normal conditions. There are too many variables and unknowns for which additional information is needed. For example, it is not known whether horizontal transmission between sheep is as important as transmission by infected feed. A first conclusion is that there is an urgent need to begin collecting the information required for an adequate assessment of the likely prevalence of BSE in small ruminants. The SSC considers that a number of possible scenarios could be listed, ranging from the situation that the exposure to infected feed is considered to be unlikely, to the scenario that BSE in small ruminants has been confirmed under natural conditions and that there are related outbreak(s) of cases over a wide geographic area. The SSC considers the SRM list proposed in its opinion of April 2000 to be valid for the time being. When more comprehensive data on BSE infectivity in sheep is available, the SSC will revisit this conclusion. The SSC proposes that rapid tests should be developed for differential diagnosis of TSEs in small ruminants. There is further a need to significantly improve surveillance and culling strategies and to accelerate the introduction of a system of individual identification of sheep and of criteria for the determination of the TSE-free status of flocks. A start must be made for the collection of data useful for the assessment of the risk of BSE in small ruminants at the national level, but also at the level of management types and other local features (e.g., farming practices). The SSC further recommends EU wide national programmes of breeding towards a genetically fully resistant sheep population. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

3 2 2. The SSC pre-emptively lists a number of additional measures that will need to be considered should there be strong indications or evidence of BSE actually being present in domestic flock. These refer to specified risk materials, culling strategies and generalised testing of animals. The detailed scientific report attached to the full opinion provides an update of the state of affairs regarding tissue infectivity distribution on the basis of the most recent results of the ongoing experiments on BSE in small ruminants. They seem to indicate that BSE, experimentally transmitted to sheep, is likely to show a disease development comparable to scrapie in sheep. This would imply that the list of specified risk materials that must be excluded from consumption, should BSE in small ruminants be likely, would be more comprehensive than in cattle. 3. Regarding the criteria to be fulfilled to conclude that the BSE/vCJD strain has been isolated from small ruminants and the possible positive identification of BSE in small ruminants, the SSC considers that, at the moment, the complete protocol of strain typing by mouse bioassay experiments is the only reliable method for detecting/confirming BSE infectivity in sheep. This protocol is long and presently takes 2 years to be completed. No other clinical or pathological features can conclusively confirm the presence of BSE although there are preliminary indications that some new tests may be able to do so. The SSC points out that a tentative list of currently available tests and tests under development was presented in the SSC Opinion on Criteria for the diagnosis of clinical and pre-clinical TSE disease in sheep, adopted on April \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

4 3 I. TERMS OF REFERENCE On September 1998 the Scientific Steering Committee (SSC) adopted its opinion on The risk of infection of sheep and goats with Bovine Spongiform Encephalopathy agent. and on April 2000 its opinion on Specified risk materials of small ruminants. Following the adoption of these opinions, the European Commission requested the SSC to (1) pre-emptively carry out risk assessments corresponding to a range of circumstances (scenarios) should BSE 1 be confirmed to occur in the sheep population under natural conditions,(2).present an updated state of affairs regarding tissue infectivity distribution on the basis of the most recent results of the ongoing experiments on BSE in small ruminants and (3) identify and list the criteria to be fulfilled to conclude that the BSE/vCJD strain has been isolated from sheep. The request is a logical follow-up of a number of SSC opinions indicating that, although there is currently no evidence to suggest that BSE occurs naturally in sheep and goats under field conditions, there are a number of reasons for concern, for example: - the difficulties in relation to surveillance and eradication of scrapie/bse in sheep; - the risk of infection of small ruminants with BSE; - the uncertainties related to breeding sheep to be resistant to scrapie/bse; - the difficulties in diagnosing clinical and pre-clinical TSE disease in sheep and to differentiate TSE agent strains. The present opinion is largely based on an in-depth evaluation by the TSE/BSE ad hoc Group of a detailed report prepared by a special Working Group and subsequent discussion by the Scientific Steering Committee at its meeting of 8-9 February The report is attached to this opinion. Scope of the present opinion: Annex 1 provides the background to and history leading to the submission to the SSC of the above questions. For the purpose of the present opinion, small ruminants includeonly sheep and goats. Because much more experimental and field information is available for sheep, the assessment has been initially and primarily carried out for sheep. However, the conclusions will be broadened to goats by taking into account whatever information is available. It is noted that only sheep and goats have been experimentally challenged and succumbed to BSE infection. Scrapie occurs naturally in sheep, less frequently in goats and moufflon. Maternal transmission (which occurs in sheep) has not been confirmed as occurring in goats. 1 Regarding the difference between presence of a PrP(res) in a tissue and this tissue also being infectious: see the SSC opinions of April 2000 on Oral exposure of humans to the BSE agent: infective dose and species barrier and on The Safety of ruminant blood with respect to TSE risks \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

5 4 It is further noted that the agent causing scrapie, the expression of clinical disease in scrapie-affected sheep, cannot currently be distinguished from BSE by any means other than biological strain typing of the agent responsible. In the present report, it will be assumed as a reasonable work approach that, in a given infected animal, in general, the BSE agent behaves in sheep and goats like the scrapie agents in sheep and goats. Classical scrapie is present as a persistent disease with a variable within-flock incidence. III. III.1 ANSWERS TO THE QUESTIONS Following analysis and discussion of the attached detailed report, the Scientific Steering Committee proposes the following answers to the 3 questions: Pre-emptive risk assessment corresponding to a range of circumstances (scenarios) should BSE be confirmed to occur in the sheep population under natural conditions. Preamble: The Scientific Steering Committee considers that it cannot at this stage assess the risk to the human population using the range of circumstances presented in the detailed report. There are too many variables and unknowns for which additional information would be needed. There is not a precise diagnostic procedure widely available to detect BSE in small ruminants. Data is lacking on the likely prevalence of exposure to the likely source of infection, the quality of surveillance for both TSE and BSE in the affected population and the actual occurrence of sheep to sheep transmission of BSE if this occurs. The SSC considers that its opinion of September 1998 regarding BSE in sheep remains valid namely that one "has to assume that BSE could have been introduced into the sheep and goat population because it has clearly been demonstrated [by experiments] that BSE can be orally transmitted to certain genotypes of small ruminants and because it is likely that BSE-contaminated MBM has been fed to some sheep and goats". There is no evidence to confirm that BSE is present in small ruminants. However, the extent of observations to detect its presence is very limited and we do not at present have adequate means (by monitoring, testing, etc.) to confirm a diagnosis of BSE in sheep directly or indirectly exposed to the BSE A number of possible scenarios could be addressed, ranging from the situation that the exposure to infected feed is considered to be unlikely, to the scenario that BSE in small ruminants would have been confirmed under natural conditions and that there are outbreak(s) of cases with epidemiological links between each other over a whole geographic area. The identification of definitive BSE in sheep or goats (if based on biological strain typing) is likely to be retrospective by a number of years. In the meantime a level of uncertainty and potential or actual risk may exist until the new situation is assessed and new measures to control any actual or potential risk are in place and effective. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

6 5 Several SSC opinions already contain recommendations with regard to the risk should BSE in small ruminants be present and to avoid exposure of small ruminants to infectivity. In addition the SSC wishes to offer a number of recommendations. III.1.1.Diagnosis of BSE in small ruminants. The SSC considers that the current numbers of TSE cases in small ruminants that are submitted to a diagnostic test or assay for BSE is far too limited as compared to the total number of small ruminants in the European Union. In fact, currently only 178 isolates from UK sheep have been inoculated into mice (for biological strain typing) and an additional 12 isolates from other European countries have been inoculated giving a total of 190 isolates that have been inoculated in UK. Results so far indicate that no BSE agent was present. The SSC recommends: That a sufficient number of testing facilities to carry out these tests is guaranteed; That research efforts are undertaken to further develop and validate possible other tests that are more rapid; That the number of such tests is drastically increased in all EU countries where scrapie occurs so as to permit reliable conclusions to be possible; That the laboratories that have the appropriate expertise, protocols and material share these with other laboratories in other countries; That a "ring-test" programme be launched to guarantee, whenever possible, harmonisation between laboratories. III.1.2.Scenario: BSE in small ruminants is not excluded. Even if BSE is not found in small ruminants, the SSC recommends the following actions, (some of them are already being applied fully or partly in some Member States): With respect to human exposure protection: Specified risk materials. The SSC refers to the SSC opinions of April 2000 listing the specified risk materials in small ruminants. In that opinion, it considered that: "at present, in the absence of evidence that BSE is present in any national small ruminant flock: - The list of tissues and materials that possibly pose the highest risk includes: skull (including the brain, pituitary gland, dura mater, eyes and tonsils) and spinal cord of all small ruminants above 12 months and spleen of small ruminants of all ages.( ). - The potential infectivity of the intestine and lymph nodes of sheep (experimentally) infected with BSE is underway and results are urgently needed to improve the risk assessment." Meanwhile, some incomplete additional results on PrP(res) distribution in tissues as well as for infectivity have become available (See Annex 2). The SSC considers the SRM list proposed in its opinion of April 2000 to be valid for the time being. When \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

7 6 more comprehensive data on BSE infectivity in sheep is available, the SSC will revisit this conclusion. Culling. The SSC recommends a culling programme to be implemented. Culling schemes should basically lead to two effects: 1. a reduction of future number of clinical cases; 2. a reduction of present prevalence of sub-clinical infection, which would reduce the human exposure risk to products derived from infected animals. Various options may be considered to achieve these goals. Some are illustrated in the attached detailed report. Tracing and certification. The SSC recommends the acceleration of the introduction of a system of individual identification of sheep (ideally to enable tracing of the parents). The SSC also recommends that criteria for the determination of scrapie- and TSE-free status of small ruminant flocks be developed. With respect to general measures: Surveillance: - Introduction of an improved surveillance system, composed of the following two components facilitating the quantitative estimate of the occurrence of TSE in small ruminants: a. TSE-in-small-ruminants surveillance that includes active surveillance of potentially exposed populations, awareness campaigns, etc., as outlined in the attached report. b. After validation of available rapid post mortem tests for TSE in small ruminants, testing of defined tissue (retropharyngeal lymphnodes, tonsils, other) of statistically sound numbers of slaughtered animals of a specified age (to be defined according to ongoing research 2 ) from selected populations, for the presence of a TSE agent. - Accelerate the development and validation of new rapid TSE tests and of differential BSE/other TSE diagnostic tests in small ruminants, both for confirmatory purposes and to verify the presence of PrP-res in live animals. - Strain typing by mouse bio-assay for at least one isolate from as many incidents of TSE in small ruminants as possible and reasonably feasible, especially if the genotype of the sheep codes for resistance. - Start a programme of genotyping of all small ruminants (giving priority to the populations considered at the highest potential risk). Risk assessment. Start the collection of data useful for the assessment of the risk of BSE in small ruminants at the national level, but also at the level of management types and other features (e.g., farming practices). It is however expected that the availability of exploitable data will be limited for most countries The assessment of the possible presence of risk of BSE in small ruminants or 2 So far, PrP(res) has first been detected in susceptible genotypes between months after inoculation at 6 months of age, in non visceral tissues and tonsils and 22 at months after inoculation in CNS tissue. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

8 7 the extent of such risk if BSE in small ruminants were confirmed, will therefore most likely and in the first place have to be based on the outcome of testing programmes with validated differential BSE: Scrapie tests and of reliable surveillance. Breeding and genotyping of sheep. The SSC recommends that projects within the EU of breeding towards resistance (meaning that BSE latency can be excluded) should be initiated, using TSE resistant rams, in accordance with the SSC opinion "Breeding for scrapie resistance" of July Information. Information campaigns on TSEs in small ruminants for all involved people. With respect to research, the SSC recommends to launch additional research where needed. The TSE/BSE ad hoc Group considers that an essential role should be played by the "TSE/BSE research network" established on 15 December 2000 by the Research Directorate General on suggestion by the Council of Ministers. III.1.3.BSE in small ruminants is likely but not confirmed If BSE in small ruminants is likely (including circumstantial evidence from any domestic flock) but not confirmed in a domestic flock, the SSC reiterates its view that the most recent data on tissue infectivity obtained from experiments on BSE in small ruminants should be assessed with a view to the possible extension of the list of tissues to be considered as specified risk materials. At the same time culling, tracing and certification should be reinforced and other pro-active actions such as those listed in section III.1.4. may be considered. III.1.4.BSE in small ruminants is confirmed Should there be evidence of BSE being actually present in any domestic flock, further actions may include: Identify which risk level is applicable on the basis of the approach outlined in the attached report. Culling/destruction of all suspects; by "suspects" is meant > the confirmed case, its offspring and other traceable relatives (including birth cohorts); > all the animals with genotype pointing at TSE-susceptibility, both in the affected and in the contact flocks. Restrict movements of the remaining resistant animals to the slaughterhouse and prohibit the use of any part of a restricted animal for human consumption until declared fit for human consumption after TSE testing by the Competent Veterinary Authority. Depending upon the extent of the risk, the culling should be applied to the whole exposed population at flock level only, or for a given management type, or possibly for a whole region (e.g. a hill area where co-grazing is practised) or country. Detailed epidemiological analysis according to the scheme specified in Annex 3. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

9 8 (If feasible: rapid genotyping of all animals in the affected flock and the flocks linked to it.) Repopulation should be done from uninfected (certified scrapie and TSE-free) flocks or with genetically fully resistant animals; (this will also reduce the exposure of sheep and man but may have impact on the reporting likelihood.) Use resistant rams for breeding in affected flocks; Possibly: disinfection of affected premises with a disinfectant appropriate for TSEs as illustrated in the detailed report. The SSC considers that the risk assessment that will be carried out should a first case of BSE in small ruminants be found should take into account the general prevalent underreporting of TSE in small ruminants. III.2. III.3. Update of the state of affairs regarding tissue infectivity distribution on the basis of the most recent results of the ongoing experiments on BSE in small ruminants, The main recent findings can be summarised as follows: BSE agent has been experimentally transmitted to susceptible sheep and goats following oral challenge. Infectivity and PrP-res have been detected in the spleen, most other lymphoreticular tissues and the CNS but variably depending on the stage of incubation and PrP genotype. A Summary of the pathogenesis of experimental BSE in sheep carrying the ARQ and ARR allele is attached as annex 2. This summary is based on the limited research results available in this field. From experiments done with sheep of the Romney breed it appears that some ARQ homozygous sheep of this breed have no detectable PrP-res in peripheral tissues after neuro-invasion has occurred. No infectivity nor PrP-res has so far been detected in any tested tissue of ARQ/ARR or ARR/ARR animals orally challenged with BSE up to 2 years post challenge. On the assumption that PrP-res and infectivity are correlated this would suggest that if these animals are infected, the titre of infectivity in a substantial period (and up to 2 years) following challenge is lower (or undetectable) in peripheral tissues of these genotypes than in more susceptible genotypes. It will not be possible to determine certainly whether these peripheral tissues in challenged animals are detectably infected or not until the experiments have reached their conclusion in several years time. Unless titrations are done the titre of infectivity (if any) will not be possible to assess. Thus a quantitative risk analysis for these tissues may not be possible unless no infectivity is detected in a statistically significant number of animals. Identification and listing of the criteria to be fulfilled to conclude that the BSE/vCJD strain has been isolated from sheep. The characteristics that could potentially be used to distinguish TSE affected animals at higher risk of being infected with the BSE agent from other TSE affected animals include: clinical presentation, PrP genotype, age, breed, type of animal (milk or meat), histopathology, biochemical analysis, flock history and potential exposures to other TSE affected flocks and infected feed. There are currently insufficient data available on the characteristics, including the clinical or pathological presentation of BSE in sheep to allow individual TSE affected animals that are more likely to be infected with the BSE agent to be identified. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

10 9 At the moment the complete protocol of strain typing by mouse bioassay experiments is the only method for detecting/confirming BSE infectivity in sheep; no other clinical or pathological features can conclusively confirm presence of BSE (i.e. distinguish between BSE and scrapie agents) although there are indications that some new tests may be able to do so. However, recent data indicate that in case of concurrent infections with scrapie strains and BSE agent, the BSE agent might be masked by scrapie agent in the mouse bioassay. A tentative list of currently available tests and tests under development is presented in the SSC Opinion on Criteria for the diagnosis of clinical and pre-clinical TSE disease in sheep and for differential biochemical determination of TSE agent strains, adopted on April In the detailed report, a list of factors which will target cases for further testing is presented and discussed. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

11 10 ANNEX 1. BACKGROUND: SUMMARY OF THE SSC OPINIONS ON TSE IN SHEEP. 1. In the Report of the Scientific Veterinary Committee on Surveillance of transmissible spongiform encephalopathies adopted 11 June It was noted that "Following the experimental oral transmission of BSE to sheep and goats there are theoretical possibilities that sheep and goats could be infected naturally via feed (MBM) by the BSE agent" In respect to surveillance for scrapie/tse in sheep a passive and active surveillance programme was recommended, taking into account that scrapie (and BSE, if BSE in sheep behaves like scrapie in sheep), is a flock disease. 2. The minimum criteria to be met by scrapie eradication programmes are described in the Report on the Standards for EC funding of Eradication Programmes for Scrapie of the Scientific Committee on Animal Health and Animal Welfare, adopted 23 June This report considered the hypothetical infection of sheep by the BSE agent and recommended that such programmes should be extended to all TSEs in small ruminants. The recommendations in the report are based on the then current scientific knowledge on scrapie but it was concluded that, if BSE were confirmed to occur in sheep, the entire flock and its contacts should be destroyed. Criteria were defined so that progress in eradication programmes should be expected, with the ultimate aim of eradicating the causative agent. These criteria may be summarised as follows: animal identification, movement recording, disease reporting, inspection of holdings, auditing of records, checking of cull animals, effective farm management, action on farm when disease confirmed, publicity about the disease, compensation payments, adequate diagnostic resources and a reference laboratory for standardisation of test techniques and strain typing. 3. In its opinion of September 1998 on The risk of infection sheep and goats with Bovine Spongiform Encephalopathy agent, the SSC concluded : "Because it has clearly been demonstrated that BSE can be orally transmitted to certain genotypes of small ruminants, and because it is likely that BSE-contaminated MBM has been fed to some sheep and goats, the Scientific Steering Committee has to assume that BSE could have been introduced into the sheep and goat population. Therefore it can not be excluded that the risk could persist, even after an effective implementation of a ruminant feed ban. On the basis of data on feeding practices, sheep and goats in many countries have probably been exposed to the BSE agent through MBM. It is noted that the feeding practices, e.g., the age and extent of MBM feeding of sheep and goats, are different from cattle. These will also vary depending on whether the animals are to be used for meat, wool or dairy purposes." 4. In its opinion of May 1999 on Actions to be taken on the basis of (1) the September 1998 SSC Opinion on the Risk of infection of sheep and goats with the BSE agent and (2) the April 1999 SEAC Subgroup report on research and surveillance for TSEs in sheep", the SSC proposed immediate actions concerning the improvement, strengthening and acceleration of the implementation in all EU Member States of an epidemio-surveillance system for TSE in sheep, and testing of a statistically representative number of sheep for the presence of TSE. Furthermore research on modelling of BSE in sheep, validation of large scale testing methodology for TSE in \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

12 11 sheep and creation of an effective network between research groups was proposed. In the medium and long term, full implementation of high quality epidemio-surveillance systems, improvement of the methods and speed of large scale applicable differential diagnosis of BSE and scrapie on post mortem samples as well as development of in vivo tests for TSE diagnosis and differential scrapie-bse diagnosis was proposed. 5. In the SSC opinion of July 1999 on "The policy of breeding and genotyping of sheep, i.e. The issue whether sheep should be bred to be resistant to scrapie, the SSC concluded that, if BSE has infected sheep, which is not certain today, and if BSE transmits and behaves in sheep in a manner similar to scrapie, for which there are some experimental indications, then a similar strategy for BSE and scrapie should be adopted. The following steps as a preliminary to the introduction of scrapie-resistance breeding programmes were recommended: - At the level of the EU: analysis of the breeding programmes that are presently ongoing in a number of European countries and establishment of a register with the resistant breeds per country and their corresponding genotypes. - At the level of each EU country: start the genotyping of large numbers of animals, in order to acquire a view on the distribution of the various genotypes in the national flocks. This would provide a first step towards the estimation of the repartition of genotypes involved in resistance against scrapie and, possibly, BSE. 6. The possibilities for TSE diagnosis in sheep were presented in the SSC Opinion on Criteria for the diagnosis of clinical and pre-clinical TSE disease in sheep and for differential biochemical determination of TSE agent strains, adopted on The SSC opinion of April 2000 on Specified risk materials of small ruminants stresses that if BSE in sheep or goats was to occur under natural conditions in a region or a country, wherever this risk occurs in an animal or flock no tissues that are likely to contain BSE infectivity should enter any food or feed chain. 8. In its opinion of October 2000 on the Implications of the Houston et al paper in The Lancet of 16 September 2000 on the Transmission of BSE by blood transfusion in sheep. (The Lancet, Vol. 356, pp ; ; 1013), the SSC concluded that "Confirmation is needed on two major points i.e. identification of the agent (BSE or not) and the origin of the transmission. Pending confirmation, the data in this experiment are new to the extent that they show that the exchange by transfusion of (400 ml of) whole blood taken during the incubation period of a sheep infected with the BSE agent can transmit BSE to a healthy sheep. This ovine model adds to data obtained in mouse and hamster models of scrapie and human TSE. As these preliminary data still lack results from the controls and do not confirm the identity of the strain (scrapie or BSE) in both the donor and recipient animals, they can only be considered a tentative evidence of the transmissibility of the BSE agent through blood." \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

13 12 ANNEX 2: EXPERIMENTAL BSE IN SHEEP: DISTRIBUTION OF INFECTIVITY BY INCUBATION STAGE AND PRP GENOTYPE AND STAGE OF INCUBATION. INFECTIVITY TITRE PRE-CLINICAL CLINICAL ARR/ARR, ARR/ARQ ARQ/ARQ ARR/ARR, ARR/ARQ ARQ/ARQ HIGH MEDIUM Spleen Lymph nodes Tonsil Brain Spinal cord Spleen Lymph nodes Tonsil LOW PrP-res DETECTED BUT INFECTIVITY NOT TITRATED spleen Lymph nodes Intestine Forestomachs abomasum Intestine Forestomachs abomasum NOT DETECTABLE Brain, Spinal cord, Spleen Lymph nodes, Tonsil Notes: The summary table is based on the limited research results currently available in this field. Full literature references are provided in the attached report. The table should be used with caution since it relates to experimental, and not natural BSE in sheep, some data are incomplete and some experiments are on-going. Nevertheless it may serve as a guide to the degrees of risk that may exist. The Table should be updated as new results come forward. No PrP-res has so far been detected in ARQ/ARR or ARR/ARR animals inoculated with BSE up to 2 years post challenge. On the assumption that PrP-res and infectivity are correlated this would suggest that if these animals are infected the titre of infectivity in the years immediately following challenge is lower (or undetectable) in peripheral tissues of these genotypes than in more susceptible genotypes. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc

14 ANNEX 3: EPIDEMIOLOGICAL ANALYSIS OF A 'BSE IN SMALL RUMINANTS'-CASE. Detailed epidemiological tracing on and tracing back should allow to define a range of probabilities in respect to the origin of the cases (maternal, direct or indirect contact, drugs or vaccines, etc). This epidemiological inquiry should include information such as: > flock data: feed history, geographical co-ordinates, distance to other flocks, flock history, veterinarian(s), sheep handlers, reproduction techniques, type of flock (meat, fibre, dairy), other species in the holding; > animal data: identification, detailed history of the case, genealogy (ascendant, descendants, collaterals, cohorts); > inventory of the holding : scheme of the holding(s) > sales (tracing on); > purchase (tracing back) > mortalities; > supplies of the holding (feed, insemination, embryos, drugs); > inquiry in the holding of origin; > cartographic analysis; > Final evaluation, integrating and combining all above elements. > Genotyping and testing for PrP-res presence of all animals in the culled herd.

15 EUROPEAN COMMISSION HEALTH & CONSUMER PROTECTION DIRECTORATE-GENERAL Directorate B - Scientific Health Opinions Unit B1 - Monitoring and dissemination of scientific opinions Scientific Steering Committee PRE-EMPTIVE RISK ASSESSMENT SHOULD BSE IN SMALL RUMINANTS BE FOUND UNDER DOMESTIC CONDITIONS. AS DISCUSSED BY THE TSE/BSE AD HOC GROUP MEETING AT ITS MEETING OF 1 FEBRUARY SUBMITTED TO THE SCIENTIFIC STEERING COMMITTEE AT ITS MEETING OF 8-9 FEBRUARY 2001

16 . TABLE OF CONTENTS I. Terms of reference II. III IV. IV.1 IV.2. IV.2.1. IV.2.2. Data on Infectivity (experimental challenge of sheep with BSE) Criteria and Diagnosis. Scenarios The range of circumstances to be considered should BSE be confirmed to occur in the sheep population under natural conditions. Basic scenarios Scenario A - Current situation: small ruminants are likely to have been exposed to the BSE agent but that there is no evidence for the scenario of the presence of BSE agent in small ruminants being probable Scenario B - there is evidence for the scenario of the presence of BSE agent in small ruminants being probable IV.2.3. BSE in small ruminants has been confirmed (Scenarios C, D or E) V V.1.1 V.1.2 V.1.3. V.1.4. V.1.5. VI. VII. Annex 1: Annex 2 Annex 3 Annex 4 Annex 5 Annex 6 Annex 7 Annex 8 Annex 9 Annex 10 Discussion of some risk reducing measures other than excluding risk tissues from the feed and food chains Culling Surveillance and flock and animal movements Individual identification or labelling of all sheep Use of genetics Environmental cleaning. Acknowledgements Literature: Experimental challenge of sheep with BSE agent: tissue infectivity and genotype. Table: Natural and experimental scrapie in sheep and goats Table: Experimental BSE in sheep: distribution of infectivity by incubation stage and PrP genotype and age/stage of incubation. Current Information on transmission Genetic information on BSE in sheep Criteria and Diagnosis. Stability of agent strains Some considerations on surveillance for BSE in sheep. Some specific elements of a scheme with respect to monitoring the risk of introduction and spread of BSE by movement of flocks and animals Environmental cleaning: Some exemples \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc 1

17 . I. TERMS OF REFERENCE On September 1998 the Scientific Steering Committee (SSC) adopted its opinion on The risk of infection of sheep and goats with Bovine Spongiform Encephalopathy agent. and on April 2000 its opinion on Specified risk materials of small ruminants. Following the adoption of these opinions, the European Commission requested the Scientific Steering Committee to (1) present an updated state of affairs regarding tissue infectivity distribution on the basis of the most recent results of the ongoing experiments on BSE in small ruminants, (2) identify and list the criteria be fulfilled to conclude that the BSE/vCJD strain has been isolated from sheep and (3) pre-emptively carry out risk assessments corresponding to a range of circumstances (scenarios) should BSE be confirmed to occur in the sheep population under domestic conditions. The request is a logical follow-up of a number of SSC opinions indicating that, although there is currently no evidence to suggest that BSE occurs naturally in sheep and goats under field conditions, there are a number of reasons for concern, for example: - the difficulties in relation to surveillance and eradication of scrapie/bse in sheep; - the risk of infection of small ruminants with BSE; - the uncertainties related to breeding sheep to be resistant to scrapie/bse; - the difficulties in diagnosing clinical and pre-clinical TSE disease in sheep and to differentiate TSE agent strains. Scope of the present report: Small ruminants include, for the purpose of the present report, only sheep and goats. Because much more experimental and field information is available for sheep, the assessment has initially and primarily been carried out for sheep. However, the conclusions will be broadened to goats by taking into account whatever information is available. It is noted that only sheep and goats (of small ruminants) have been experimentally challenged and succumbed to BSE infection. Scrapie occurs naturally in sheep, less frequently in goats and moufflon. Maternal transmission (which occurs in sheep) is not confirmed to occur in goats. It is further noted that infectivity with the agent causing scrapie, the expression of clinical disease in scrapie-affected sheep, cannot currently be distinguished from BSE by any means other than biological strain typing of the agent responsible. In the present report, it will be assumed as a reasonable work approach, in general, that the BSE agent behaves in infected sheep and goats like the scrapie agents in sheep and goats. Classical scrapie is present as a persistent disease with a variable within-flock incidence. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc 2

18 . II. DATA ON INFECTIVITY (EXPERIMENTAL CHALLENGE OF SHEEP WITH BSE AGENT) BSE agent has been transmitted to both sheep and goats by the i/c and by the oral routes (Foster, Hope and Fraser, 1993). Furthermore, in contrast to cattle where BSE infectivity has not been recorded in the spleen even after i/c inoculation of cattle (GAH Wells, personal communication), sheep with experimental BSE do show such infectivity (Foster et al, 1996). This suggests that the pathogenesis of BSE in sheep (and possibly goats) may align more with the pathogenesis of scrapie in these species than with that of BSE in cattle. There is further support for this notion derived from recent studies (Jeffrey et al, 2001a; 2001b) of the distribution of PrP-res in lymphatic tissues of sheep experimentally infected with the BSE agent. In regard to consumers, the risk tissues for natural BSE agent in sheep and goats, if it occurred, would be similar to the scrapie-infected tissues. These tissues are listed by the WHO (WHO, 1997) and the SSC (SSC Opinion of April 2000 on Specified risk materials of small ruminants.) Annex 1 elaborates further on tissue infectivity and genotype. Annex 2 provides a summary classification of tissues by infectivity status in pre-clinical and clinical cases of natural and experimental scrapie in sheep and goats. Annex 3 summaries, for experimental BSE in sheep, the current knowledge on the distribution of infectivity by incubation stage and PrP genotype and age/stage of incubation. The main recent findings can be summarised as follows: BSE agent has been experimentally transmitted orally to susceptible sheep and goats, infectivity is detected in spleen, most other lymphoreticular peripheral tissues and the CNS but variably depending on the stage of incubation and PrP genotype. A Summary of BSE in sheep pathogenesis results in sheep carrying the ARQ and ARR allele is attached as annex 3. However, some ARQ homozygous sheep have no detectable PrPres in peripheral tissues after neuro-invasion has occurred. No PrP-res has so far been detected in ARQ/ARR or ARR/ARR animals inoculated with BSE up to 2 years post challenge. On the assumption that PrP-res and infectivity are correlated this would suggest that if these animals are infected, the titre of infectivity in the years following challenge is lower (or undetectable) in peripheral tissues of these genotypes than in more susceptible genotypes. It will not be possible to determine certainly whether these peripheral tissues in challenged animals are detectably infected or not until the experiments have reached their conclusion in several years time. (NOTE: unless titrations are done the titre of infectivity (if any) will not be possible to assess. Thus a quantitative risk analysis for these tissues may not be possible unless no infectivity is detected.) III. CRITERIA AND DIAGNOSIS. The characteristics that could potentially be used to distinguish TSE affected animals at higher risk of being infected with the BSE agent from other TSE affected animals include clinical presentation, PrP genotype, age, breed, type of animal (milk or meat), pathology, biochemical analysis, flock history and potential exposures to other TSE affected flocks and infected feed. There are currently insufficient data available on the characteristics, including the clinical or pathological presentation of BSE in sheep to \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc 3

19 allow individual TSE affected animals that are more likely to be infected with the BSE agent to be identified. At the moment the complete protocol of strain typing using various inbred mouse strains (or possibly using transgenic bovine mice in the future) is the only method for detecting/confirming BSE infectivity in sheep 3 ; no other clinical or pathological features can conclusively confirm presence of BSE (i.e. distinguish between BSE and scrapie agents) although there are indications that some new tests may be able to do so. However, recent data (Baron and Biacabe, 2001; see Annex 6) indicate that in case of concurrent infections with scrapie strains and BSE agent, the BSE agent might be masked by scrapie agent in the mouse bioassay. A tentative list of currently available tests and tests under development is presented in the SSC Opinion on Criteria for the diagnosis of clinical and pre-clinical TSE disease in sheep and for differential biochemical determination of TSE agent strains, adopted on April It is noted that biological strain typing using in-bred strains of mice is effective at distinguishing the BSE agent from scrapie strains but that the method is not practical for routine use. On the criteria for differential molecular/biochemical aetiological diagnosis of TSE disease in sheep it was concluded that conclusive differential biochemical strain identification is not yet possible. Fragment size and glycoform analysis adds to prion characterisation although on its own it is not sufficient for strain typing TSE isolates from sheep. Indeed, no current test is able to conclusively define strains of TSE agents. Factors which will target cases for further testing The most useful criteria for targeting natural TSE in small ruminant cases that may be BSE from scrapie cases, is the occurrence of BSE or scrapie in the region and the exposure history of the flock in which the case is found. For example, if scrapie is found in animals that could have been exposed to BSE-infected material and are in a closed flock, in a region where scrapie has not previously been identified, it would be worth targeting affected animals in this flock for mouse bioassay. If any characteristics that could distinguish TSE-affected animals that may be infected with BSE from other TSE-infected animals are identified these characteristics could be used to target control policies to the most appropriate animals. The main factors that would target a TSE case for further testing for BSE are: 1. The flock history with regard to TSE (especially if scrapie-like disease is found in a BSE region where scrapie previously did not occur), including also flock and animal movements and flock management. 2. The feeding history and practices of the small ruminant (flock) in which TSE BSE is found; Annex N 6 elaborates further on the above factors.. 3 It cannot be excluded that there exists a scrapie strain with the same transmission characteristics as BSE. This would be picked up by the complete mouse bio-assay protocol, which includes the transmission characteristics (e.g., incubation period), the distribution of lesions in the brain and the glycoform pattern. \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc 4

20 . The SSC opinion, adopted on April 2000, provides a comprehensive report on the criteria for diagnosis of clinical and pre-clinical TSE disease in sheep and for differential biochemical diagnosis of TSE agent strains is available as an. IV. SCENARIOS To date, no BSE has been diagnosed in domestic flocks of small ruminants. However, on experimental grounds and because possibly contaminated MBM has been fed in the past to small ruminants, the possibility that this could occur cannot be excluded. An initial list of sources representing the highest levels of possible risk has therefore been identified in various SSC opinions, the control of which would significantly reduce the possible exposure of humans to the BSE agent should BSE in small ruminants be present but undiagnosed. IV.1. THE RANGE OF CIRCUMSTANCES TO BE CONSIDERED SHOULD BSE BE CONFIRMED TO OCCUR IN THE SHEEP POPULATION UNDER NATURAL CONDITIONS. The two principal questions to be addressed are: the likely risk of infection being transmitted to man if BSE is found in sheep; How this risk could be reduced. The first of these questions includes consideration of the strength of the species barrier between sheep and man for the BSE agent, the likely prevalence and distribution of BSE infection in sheep, the likely duration of the epidemic in sheep and the level of infection likely to be present in various sheep tissues. The second includes consideration of the likely effectiveness, compliance with and enforcement of control measures. IV.1.1 The major determinants of risk to humans are: The current prevalence of BSE infection in sheep The likely future prevalence of BSE infection in sheep (is it increasing, reducing or being maintained at the same level) The extent to which the human population is exposed to the infection in sheep In a given situation there are various facts that need to be taken into account in assessing the likely current and future prevalence of infection in sheep and the likely exposure of the human population to this source of infection. There are some facts that are not dependent on the particular situation. These will apply in all situations and the level of knowledge about these facts will determine how accurately we can assess the risk to humans, these scenario-independent facts include; Routes and rates of transmission and what determines how rates of transmission vary Distribution of infection in tissues by species, genotype and stage of incubation Efficiency of diagnostic tests used by species, genotype and stage of incubation The nature of genotype-regulated susceptibility to BSE in sheep There are other scenario-dependent facts and those which should be considered when trying to assess the three major determinants of risk for humans listed above: \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc 5

21 . IV.1.2 Facts that should be taken into account when assessing the likely current prevalence of BSE infection in sheep. The prevalence of the likely source of infection for cases of BSE in sheep (detailed epidemiological investigation of affected flocks should be used to determine the likely source of infection for sheep); If BSE in sheep is found: the number of BSE affected flocks/animals detected; The effectiveness of surveillance in regions where BSE is detected in sheep (both the efficiency of surveillance for detecting TSE in sheep and the efficiency of surveillance to determine whether the TSEs detected in sheep are scrapie or BSE should be considered. The efficiency of surveillance for TSE in sheep will depend on farmer awareness, number of neurological submissions, consequences of reporting, diagnostic tests used, identification of sheep. The efficiency of surveillance for determining whether TSE in sheep is BSE will depend on number of animals tested and efficiency of tests). In summary the likely prevalence of BSE infection in sheep will be higher if: a) the prevalence of exposure of sheep to the likely source of BSE in sheep is high; b) the number of BSE affected flocks/animals detected at the same time is high; c) the efficiency of surveillance for TSE in sheep was/is low, because as a result the number of secondary cases may have increased. IV.1.3 Facts that should be taken into account when assessing the likely future prevalence of BSE in sheep. The following elements need to be considered when assessing the likely future prevalence of BSE in sheep: The likelihood of transmission within affected flocks is likely to depend partly on the management within affected flocks. How accurately this likelihood in any particular scenario can be assessed will depend on the level of knowledge of scenarioindependent information i.e. the routes and rates of transmission in different circumstances. The likelihood of transmission between affected flocks which depends on movement of sheep (or other contaminated or infected materials, including feed) from affected to other farms, contact with sheep from affected farms and the density of the sheep population. The genotype distribution (hence the genotype regulated level of resistance) in the affected population. This distribution is scenario-independent. (Note: One could imagine a region where the genetic susceptibility of flocks/individuals is high and another region where it is low; these would then become two separate scenarios that could significantly affect the risk and incidence of transmission to man.) The presence, date of introduction and level of enforcement of MBM in feed and SRM bans (including the likelihood of cross-contamination) Effective identification and destruction of clinically suspect animals, effective rendering controls and effective SRM bans are beneficial in reducing risk to small ruminants if a feed ban is only partially effective, but scientifically become much less important once a feed ban is completely effective since everyone agrees that the most likely route of initial exposure is via MBM. It is therefore important to know the date that a feed ban was \\DG24-SVR-01\common\WebDev\Scenarios_Sheep_OPINION_0102_FINAL.doc 6